For example, a bridge circuit such as an inverter includes multiple power devices (semiconductor switching devices) such as power MOSFET and IGBT. These power devices, when driven, indicate different source potentials at high and low sides. A gate drive circuit corresponding to either side needs to be isolated. Generally, a photo-coupler may be provided for a signal line to electrically insulate the signal line. The power supply for the gate drive circuit is provided as a floating power supply to isolate the gate drive circuit.
A pulse transformer may be used for a gate drive circuit to isolate a gate signal from the drive circuit at a time. As disclosed in JP-A-2004-194450 (see FIG. 1), for example, two gate drive power supplies and zener diodes are used to provide the configuration capable of implementing a PWM control (positive/negative asymmetric signal control) that is unavailable to ordinary configurations using pulse transformers.
Power switching devices such as power MOSFETs are requested to be controlled so as to be normally off in order to ensure safety during operation. Compared to silicon MOSFETs, next-generation devices such as an SiC (silicon carbide)-J (junction) FET and a GaN (gallium nitride) FET can greatly reduce losses and enable normally-off operation. However, many of these FETs apply just several voltages (approximately 2 V to 3 V) while an Si power MOSFET can apply a voltage of 10 V to 20 V to the gate. Such devices cannot be driven at a high voltage and therefore make high-speed switching difficult.
JP-A-2004-194450 is hardly applicable to the configuration of a bridge circuit using the above-described power devices. This is because a zener voltage of the zener diode regulates the secondary voltage of the pulse transformer, putting limitations on decrease in the gate voltage. In addition, two gate drive power supplies are needed for the primary side of the pulse transformer, requiring more costs than ordinary drive circuits.